[BRIEF NOTE] On tightly packed planetary systems

Less prominent than yesterday’s announcement of the discovery of five planets at Tau Ceti, including one potentially in that star’s habitable zone was news of a reanalysis of data concerning planets orbiting the red dwarf star Gliese 667c. News earlier this year suggested one superterrestrial planet orbited that trinary component within its habitable zone; a controversial reanalysis suggests that three might.

A Paul Gilster post at Centauri Dreams, “Tightly Spaced Habitable Zone Candidates”, notes the reanalysis, and suggests that so far as solar systems go, our system might actually have fewer planets orbiting in the habitable zone than others.

Philip Gregory (University of British Columbia) has performed a re-analysis of the HARPS data on Gl 667C that is getting play in the press because it identifies not one but three planets in the habitable zone. The star has about a third of the mass of the Sun, so according to Gregory’s figures, a habitable zone planet there produces seven times the radial velocity signature that a similar planet around a G-class star would generate. In 2011 Gl 667C was already known to be orbited by at least one planet, Gl 667C b, with a 7.2 day orbit, and there was evidence for other worlds. Later work confirmed the planet Gl 667C c in a 28-day orbit in the habitable zone.

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The result: The detected signals include the already established planets in 7.2 and 28.1-day orbits, but also show possible planets in 30.8 (d), 38.8 (e), 53.2 and 91.3-day orbits (f). Gregory discounts the 53.2-day signal because it seems to be the result of surface activity on the star. All these candidates are more massive than Earth, but e is only 2.4 Earth masses. The signals at 30.8 and 38.8 days, if confirmed, would join Gl 667C c as planets in the habitable zone. Gregory finds that the 91.3 day orbit would take that planet inside the outermost edge of the habitable zone, although its eccentric orbit would keep it outside the HZ for the majority of time.

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The scientist is careful to note that new simulations will be needed to determine which of the planetary signals are consistent with a stable planetary system. Look particularly at the closeness of the 28.1 and 30.8 day orbits, where the semi-major axis differs by a mere 0.007 AU. This sets up a closest approach, as Gregory notes, every 323 days, doubtless a fascinating astronomical spectacle from the surfaces of these possible worlds. And the author cites the Kepler mission’s own findings of systems with close planetary separations, including Kepler 36 b and c (0.014 AU), Kepler 42 b and d (0.0038 AU), and KOI 55b and c (0.0016 AU).

We may have to start getting used to solar systems with close planetary separations, unlike the relatively spacious inner system we see around the Sun. I’m reminded of something Steve Vogt (UC-Santa Cruz) said in the news release on the Tau Ceti story: “We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than one hundred days. This is quite unlike our own solar system where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.” Vogt was not speaking of M-dwarfs, of course, but the statement has no better illustration than Gl 667C’s possible planets.